Polyamide fiber material like nylon carpeting is susceptible to staining by natural and artificial coloring pigments contained in numerous food and beverages. Thus, when food and beverages are accidentally spilled onto polyamide fiber, the polyamide fiber ends up being stained by the coloring pigment contained in those food and beverages. These stains cannot easily be removed under ordinary laundering conditions. Several efforts have been made thus far to deal with this shortcoming of polyamide fiber. For example, polyamide fibers are treated with fluorine-based or silicone-based compounds. Examples of processes used commercially include adsorbing the formaldehyde condensate of a phenolic compound such as bisphenol S, a sulfonide of bisphenol S or phenolsulfonic acid onto the fiber either by immersion treatment or steaming following application by padding, spraying, or foam coating. There are also processes wherein fluorine-based compounds are applied using similar methods after the initial adsorbing process described above. Although staining by pigment contained in food and beverages can definitely be prevented, according to these processes, when these types of formaldehyde condensates of phenolic compounds are exposed to environments containing acidic gas such as nitrogen oxide gas, the formaldehyde condensates have the shortcoming of turning yellow on the fibers. In addition, when formaldehyde condensates of phenolic compounds are exposed to light, they also have the shortcoming of turning yellow on the fibers. Thus, no satisfactory method exists at present for protecting polyamide fiber from coloring pigment contained in food and beverages.